Easy:

1. Build a quartz crucible inside an inert argon atmosphere

2. Heat it to 2600F/1400C

3. Melt your highly pure silicon in it

4. Add in a seed at the top that the rest of the silicon can crystalize around

5. Now comes the tricky part that requires some experimentation: Pull out the seed very carefully while rotating it, so that an ingot forms around it as you keep pulling